Ultraviolet sensors have been used as flame sensors in fire alarms, devices for monitoring a condition of combustion in burners, or the like and further have been used as ultraviolet-ray sensing devices in simple apparatuses for measuring an amount of irradiation of ultraviolet ray outdoors. Furthermore, such ultraviolet sensors are expected to become next generation of optical communication devices using ultraviolet rays.
As ultraviolet sensors, typically, sensors using a diamond semiconductor or a SiC semiconductor have been developed and commercialized. However, ultraviolet sensors using such a diamond semiconductor or a SiC semiconductor have problems that the materials cannot be easily processed and that the cost of the materials is high.
Therefore, recently, ultraviolet sensors using an oxide semiconductor material have been studied. For example, a diode type sensor having a heterojunction structure composed of an n-type semiconductor layer including titanium oxide as a main component and a transition metal oxide thin film, the diode type sensor having sensitivity in an ultraviolet region has been proposed (refer to, for example, Patent Document 1). In the ultraviolet sensor described in Patent Document 1, the n-type semiconductor layer including titanium oxide as a main component is a single crystal substrate and the transition metal oxide thin film is formed by epitaxial growth. Note that although the single crystal substrate and the thin film are heat-treated by annealing or the like, they are not sintered bodies. Furthermore, in the ultraviolet sensor described in Patent Document 1, the transition metal oxide thin film is positioned at a light-receiving side which is irradiated by ultraviolet rays.
The ultraviolet sensor described in Patent Document 1, however, has problems as described below.
The n-type semiconductor layer including titanium oxide as a main component and placed at a base side of the ultraviolet sensor has a high specific resistance, or a low electrical conductance, so that sensitivity of the ultraviolet sensor is relatively low. This is supported by the data shown in FIG. 5 of Patent Document 1. Furthermore, since the transition metal oxide thin film is positioned at a light-receiving side, a light transmittance thereof is low. Also, this leads to the sensitivity thereof being low.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-172166.